1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus for forming a toner image on a print medium with a developer composed of toner and carriers.
2. Description of Related Art
As a conventional image forming apparatus, for example, an image forming apparatus disclosed by Japanese Patent Laid-Open Publication No. 2009-98593 is known. The image forming apparatus adopts a DC development method, wherein development is performed by applying a DC voltage between a developer support member and an image support member. In the image forming apparatus disclosed by Japanese Patent Laid-Open Publication No. 2009-98593, since the DC development is adopted, it is not necessary to apply an AC voltage between the developer support member and the image support member. Therefore, the structure of the image forming apparatus can be simple.
However, the image forming apparatus adopting the DC development method has a problem that toner images formed thereby are more prone to density unevenness than toner images formed by image forming apparatuses adopting an AC development method.
In the AC development method, generally, a voltage with an amplitude of about 700V is applied between a developer support member and an image support member. In this case, since a relatively high voltage is applied between the developer support member and the image support member, a relatively large amount of toner contributes to development. Therefore, in an image forming apparatus adopting the AC development method, even if the gap between the developer support member and the image support member fluctuates due to non-uniform rotations of the developer support member and the image support member, it is less likely that toner images formed thereby have density unevenness.
In the DC development method, on the other hand, a DC voltage of about 150V is applied between a developer support member and an image support member. In this case, since a relatively low voltage is applied between the developer support member and the image support member, only a relatively small amount of toner contributes to development. Therefore, in an image forming apparatus adopting the DC development method, if the gap between the developer support member and the image support member fluctuates due to non-uniform rotations of the developer support member and the image support member, toner images formed thereby are prone to density unevenness.
The image forming apparatus also adopts a counter development method. In the counter development method, the developer support member and the image support member rotate in the same direction, whereby the developer support member and the image support member travel in the opposite direction at a position to face to each other. In the image forming apparatus disclosed by Japanese Patent Laid-Open Publication No. 2009-98593, since the counter development method is adopted, more toner comes into contact with an electrostatic latent image formed on the image support member. Therefore, it is less likely that toner images formed by the image forming apparatus have density unevenness.
However, the image forming apparatus disclosed by Japanese Patent Laid-Open Publication No. 2009-98593 has still the following problem. Since the developer support member and the image support member travel in the opposite direction at a position to face to each other, carriers of the developer come into contact with the formed toner image, which may cause stripe noise in the trailing edge of the toner image.